CO2-corrosion-of-mild-steel-Trends-in-Oil-and-Gas-Corrosion-Research-and-Technologies

CO
2
corrosion of mild steel 7
Aria Kahyarian
1
, Mohsen Achour
2
and Srdjan Nesic
1
1
Ohio University, Athens, OH, United States;
2
ConocoPhillips, Bartlesville, OK, United States
7.1 Introduction
Produced oil and gas are always accompanied by some water and varying amounts of
carbon dioxide and in some cases hydrogen sulfide and organic acids. All of these may
affect the integrity of mild steel. This has been known for over 100 years, yet internal
corrosion of pipelines and other facilities made from mild steel still represents a chal-
lenge for the oil and gas industry. Although many corrosion-resistant alloys are able to
withstand this type of corrosion, it is a matter of economics: mild steel is still the most
cost-effective construction material. The price of failure due to internal corrosion is
enormous, both in terms of direct costs, such as repair/replacement costs and lost pro-
duction, and indirect costs, such as environmental cost and impact on the downstream
industries.
The following text summarizes the current degree of understanding of CO
2
corro-
sion of mild steel exposed to aqueous environments. Much has been understood about
the basic mechanisms, enabling construction of mechanistic prediction models, but
many challenges remain, particularly when it comes to effects of corrosion product
layers, multiphase flow, additional species, hydrocarbon composition, inhibition,
etc. The following sections start out with describing the basic physicochemical phe-
nomena underlying CO
2
corrosion and gradually move to more complicated situations,
culminating with the list of multifaceted real-life challenges seen in the field.
7.2 Water chemistry in CO
2
corrosion
Carbon dioxide (CO
2
) is a stable, inert, and noncorrosive gas. However, upon disso-
lution in water and a subsequent hydration reaction, a more reactive chemical species,
carbonic acid (H
2
CO
3
), is formed. This reaction is followed by dissociation reactions
to form bicarbonate HCO3ion, carbonate CO32ion, and hydrogen (H
þ
) ion,
resulting in an acidic and corrosive solution. The reactions associated with these chem-
ical equilibria and their corresponding mathematical relationship are listed in
Table 7.1.
The chemical equilibria and water chemistry associated with dissolved CO
2
and its
carbonate derivatives have been extensively studied [1e9]. The first step, CO
2
disso-
lution, is described via Eq. (7.i), where KCO2is the proportionality constant and can be
expressed as Henry’s constant for the simplest case of ideal gas and ideal solution. The
nonideal behavior of the gas phase, represented by fugacity coefficient, and the liquid
Trends in Oil and Gas Corrosion Research and Technologies. http://dx.doi.org/10.1016/B978-0-08-101105-8.00007-3
Copyright ©2017 Elsevier Ltd. All rights reserved.

phase, represented by activity coefficients, can be incorporated into the proportionality
constant following the extended Rault’s law.
The product of CO
2
hydration (reaction 7.ii), carbonic acid (H
2
CO
3
), is a diprotic
weak acid. The term weak acid refers to the fact that this species is only partially disso-
ciated in an aqueous solution. With the first dissociation constant of pK
ca
z3.6
(reaction 7.iii) and the second dissociation constant of pK
bi
z10.33 (reaction 7.iv),
the carbonic acid dissociation reaction can be considered as the main source of acidity
(H
þ
) in the solution. Although water can also be categorized as a weak acid, with
pK
a
z14, it has no significant effect when compared with H
2
CO
3
and HCO
3.
The chemical equilibria shown in Table 7.1 represent a simple case of CO
2
disso-
lution in pure water, such as what is observed in condensed water formed in wet gas
pipelines. However, a more complex water chemistry is found in formation water,
where significant amounts of various ions such as Cl
,Na
þ
,Ca
þ
,SO
2
4, organic
acid (such as acetic acid, formic acid, and propionic acid), as well as hydrogen sulfide
can be present [10,11]. These species can significantly alter the speciation of CO
2
equilibria by changing the acidity and ionic strength of the solution.
A more comprehensive discussion on the chemical speciation of CO
2
/water system
is provided in Chapter 34.
7.3 Electrochemistry of CO
2
corrosion
The aqueous CO
2
corrosion of mild steel, as seen in oil and gas industry, is by nature
an electrochemical system. The spontaneous iron dissolution, causing the deterioration
of the metallic structure, is an electrochemical oxidation process. On the other hand,
the cathodic hydrogen evolution reaction provides the required electron sink for the
iron dissolution to progress. In the CO
2
corrosion context, the hydrogen evolution re-
action is a family of cathodic reactions with all having molecular hydrogen as their
Table 7.1 Chemical reactions of acidic water/CO
2
equilibria
Reaction Equilibrium equation
CO2ðgÞ!CO2ðaqÞKCO2¼hCO2ðaqÞi
pCO2ðgÞ
(7.i)
CO2ðaqÞþH2OðlÞ!H2CO3ðaqÞKhyd ¼½H2CO3
hCO2ðaqÞi(7.ii)
H2CO3ðaqÞ!HCO
3ðaqÞþHþ
ðaqÞKca ¼HCO
3½Hþ
½H2CO3
(7.iii)
HCO
3ðaqÞ!CO2 3ðaqÞþHþ
ðaqÞKbi ¼CO2
3½Hþ
HCO
3
(7.iv)
H2OðlÞ!OH
ðaqÞþHþ
ðaqÞKw¼½OH½Hþ(7.v)
150 Trends in Oil and Gas Corrosion Research and Technologies

product. That includes the reduction of H
þ
,H
2
CO
3
, HCO
3, and H
2
O, as shown in
Table 7.2. Considering the chemical equilibrium of the water/CO
2
system, it can be
shown that all these reactions are thermodynamically identical. This means that they
have the same reversible potential (based on Nernst equation), if the concentrations
of the involved chemical species are defined by the equilibrium speciation. Hence,
the main difference resides in reaction kinetics. The same would also hold true for
other weak acids, such as acetic acid and hydrogen sulfide.
Table 7.2 summarizes the commonly accepted electrochemical reactions associated
with aqueous CO
2
corrosion of mild steel. Reactions (7.vi)e(7.ix) are the hydrogen
evolution reactions in a water/CO
2
solution. Reactions (7.vi) and (7.vii) are the
hydrogen ion and water reduction reactions, respectively. Reaction (7.viii) is the
reduction of carbonic acid (H
2
CO
3
), and reaction (7.ix) is the reduction of bicarbonate
ion, which is believed to be significant at near-neutral and alkaline pH values because
of the high bicarbonate ion concentration [12e15].
7.3.1 Anodic reactions
The iron oxidation as the dominant anodic reaction is a key element in acidic corrosion
of mild steel. The mechanism of iron oxidation reaction in acidic media has been the
subject of numerous studies over the last half a century [16e27] and has been proved
difficult to explain. In this section, the mechanism of acidic iron dissolution is briefly
discussed to provide the necessary context relevant to CO
2
corrosion; a thorough
review of the existing literature can be found elsewhere [26,28].
El Miligy et al. [19] showed that the iron dissolution in mildly acidic environments
occurs in four different states, depending on the electrode potential. The authors cate-
gorized these as active dissolution,transition,prepassivation, and passive, as demon-
strated in Fig. 7.1. Each range was shown to have a different electrochemical behavior,
characterized by different apparent Tafel slopes and reaction orders. The two local cur-
rent maxima, observed in transition and prepassivation ranges, were showed to be also
pH dependent. This suggests that the mechanism of iron dissolution at corrosion
potential could depend on the solution pH and other environmental conditions.
Table 7.2 Electrochemical reactions associated with aqueous acidic
CO
2
corrosion of mild steel
Electrochemical reaction Dominant reaction type
Hþ
ðaqÞþe!1
2H2ðgÞCathodic (7.vi)
H2OðlÞþe!OH
ðaqÞþ1
2H2ðgÞCathodic (7.vii)
H2CO3ðaqÞþe!HCO
3ðaqÞþ1
2H2ðgÞCathodic (7.viii)
HCO
3ðaqÞþe!CO2 3ðaqÞþ1
2H2ðgÞCathodic (7.xi)
Fe2þ
ðaqÞþ2e!FeðsÞAnodic (7.x)
CO
2
corrosion of mild steel 151

For the case of CO
2
corrosion, at pH values less than 5, the experimental results
suggest that the corrosion is occurring at the active dissolution range [29,30].AtpH
values more than 5, the corrosion potential gradually shifts towards the transition
range, and eventually reaches the prepassivation range at near neutral pH values [20].
The complex behavior depicted in Fig. 7.1 is an indication of a reaction mechanism
with multiple intermediate species and rate-determining steps. In the literature, there
are two main mechanisms proposed for iron dissolution in acidic solutions: the
“catalytic mechanism”and the “consecutive mechanism.”These two mechanisms
are associated with two distinct electrochemical behaviors observed specifically in
the active dissolution range. The catalytic mechanism, first proposed by Heusler
et al. [31], is based on the experimental Tafel slope of 30 mV and second-order depen-
dence on hydroxide (OH
) ion concentration. On the other hand, the consecutive
mechanism proposed by Bockris et al. [24] was formulated to explain the observed
Tafel slope of 40 mV and a first-order dependence on (OH
) ion concentration. These
two significantly different reaction kinetics are believed to be caused by the surface
activity of the iron electrode [17], i.e., the dissolution of cold-worked iron electrodes
with high internal stress occurs with a 30 mV Tafel slope, whereas a 40 mV Tafel
slope was observed for dissolution of recrystallized iron [17,25,27,28]. The catalytic
mechanism is described as reactions (7.xi)e(7.xiv) [28].
Fe þH2O#ðFeOHÞads þHþþe(7.xi)
Fe þðFeOHÞads #½FeðFeOHÞ (7.xii)
–500
–400
–300
–200
–100
0
100
200
300
400
0.0001 0.001 0.01 0.1 1
Potential vs. SHE (mV)
Current density (A/m
2
)
Active dissolution
Transition
Pre-passivation
Passive
Figure 7.1 Anodic polarization curve of iron in 0.5 M Na
2
SO
4
solution at pH 5 and 298K, with
the scan rate of 6.6 mV/s and rotating disk electrode at 69 rps. SHE, standard hydrogen
electrode.
Adapted from A.A. El Miligy, D. Geana, W.J. Lorenz, A theoretical treatment of the kinetics of
iron dissolution and passivation, Electrochimica Acta 20 (1975) 273e281.
152 Trends in Oil and Gas Corrosion Research and Technologies

½FeðFeOHÞ þ OH/FeOHþþðFeOHÞads þ2e(7.xiii)
FeOHþþHþ#Fe2þþH2O (7.xiv)
This mechanism suggests that the (FeOH)
ads
on the so-called kink sites acts as a
catalyst in the iron dissolution reaction. Although this mechanism has been criticized
because of the two-electron transfer step (reaction 7.xiii)[25], it has been supported
by atomistic scale discussions and electrochemical impedance measurements
[21,22,27].
The consecutive mechanism [24] shares the same initial and final step with the
“catalytic mechanism,”as shown in reactions (7.xv)e(7.xvii). However, in this mech-
anism, (FeOH)
ads
is directly oxidized through a one-electron transfer elementary step
(reaction 7.xvi).
Fe þH2O#ðFeOHÞads þHþþe(7.xv)
ðFeOHÞads /Fe½OHþþe(7.xvi)
FeOHþþHþ#Fe2þþH2O (7.xvii)
The anodic polarization curves obtained for mild steel dissolution in CO
2
-saturated
environments have frequently been reported to have a 40 mV Tafel slope and a first-
order dependence on hydroxide ion concentration [13,15,29,30,32], in accordance
with the “consecutive mechanism”proposed by Bockris et al. [24]. Hence, this mech-
anism and its corresponding kinetic relationship have been commonly used to describe
the anodic currents in CO
2
corrosion of mild steel [29,30,33e36]. On the other hand,
although a few studies report a rather significant effect of CO
2
and other carbonate spe-
cies on the acidic iron dissolution reaction [13,20], in-depth analysis on the extent of
these possible effects is rarely available in the literature. In a study by Ne si c et al. [20],
the anodic polarization curves were used to discuss the effect of CO
2
in a short poten-
tial range (w100 mV) above the corrosion potential. The experiments were performed
in perchlorate solutions, in the pH range of 2e6 and pCO
2
from 0 to 1 bar. In that
study, the authors reported the Tafel slope of 30 mV and second-order dependence
on OH
ion concentration for pH values less than 4 (corresponding to the “catalytic
mechanism”). In the pH range of 4e6, the reported Tafel slopes were gradually
increasing, whereas the dependence on OH
ion concentration was diminishing.
Ultimately, at pH w6, a Tafel slope of 120 mV with zero dependence on OH
ion
concentration was reported. The results showed that the presence of CO
2
does not
affect the observed Tafel slopes, whereas the exchange current densities were linearly
proportional to pCO
2
over the studied pH range. The authors formulated the following
mechanism to describe the effect of CO
2
on iron dissolution.
Fe þCO2#FeCO2ads (7.xviii)
CO
2
corrosion of mild steel 153